As is known to those of ordinary skill in the art, a simple form of a one-pole high pass analog filter includes a capacitor and a resistor, wherein the values of the capacitor and resistor determine a cutoff frequency (also referred to herein as a break frequency or a −3 dB point). In order to obtain a very low break frequency, for example, on the order of a few Hertz, the values of the capacitor and/or resistor must be relatively large, and the corresponding capacitor and/or resistor tend to be physically large. Being physically large, it is generally not practical to integrate the capacitor and resistor onto a common substrate, such as a silicon substrate.
As is also known, an insulated gate field effect transistor (IGFET), which is relatively small and which can be fabricated on a substrate, can be used to provide a relatively high value effective resistance, on the order of hundreds of Megohms. The high resistance IGFET device can be used as the resistor in a high pass filter. Because the effective resistance of the IGFET device can be quite large, the IGFET device is suitable for use in a high pass filter having a low break frequency.
Because an IGFET device, or simply IGFET, has a resistance that is non-linear in parts of its operating voltage range, a variety of techniques have been developed to provide linear operation, i.e., reduced distortion, of a high pass filter using the IGFET device.
Referring to FIG. 1, a prior art integrator circuit 10 uses IGFET devices 12, 14 and capacitors 16, 18 coupled in a differential arrangement about an operational amplifier 20. While a complete high pass filter is not shown, one of ordinary skill in the art will understand that a high-pass filter can be implemented by means of an integrator, where the integrator uses resistors and capacitors. The integrator circuit 10 uses of IGFET devices as equivalent resistors. The IGFET device 12 in combination with the capacitor 16 provides a first integrator portion and the IGFET device 14 in combination with the capacitor 18 provides a second integrator portion, which together provide an integrator transfer function between input terminals 22a, 22b and output terminals 24a, 24b. 
As is known, the value of a drain-source voltage, VDS, applied between a drain and a source of an IGFET device tends to affect its resistance in proportion to the value. An AC input voltage, Vin, applied as opposing phase signals, +Vin/2, −Vin/2, to the input terminals 22a, 22b provides opposing voltages at the source 12a of the IGFET device 12 and the source 12b of IGFET device 14, and therefore, drain-source voltages moving in opposite directions at each of the two IGFET devices 12, 14. It will be understood by one of ordinary skill in the art that the opposing voltages applied to the sources of the IGFET devices 12, 14 tend to result in opposite changes in effective resistance of the IGFET devices 12, 14 as the input AC voltage, Vin, changes in each cycle. These canceling effects occur when the input voltage, Vin, is sufficiently small so as to keep an operating point of the IGFET device 12, 14 within a parabolic range, which is further described in conjunction with FIG. 3. Therefore, when the input voltage, Vin, is relatively small, the integrator 10 provides a substantially linear transfer function or response between the input terminals 22a, 22b and the output terminals 24a, 24b. 
However, as is known, larger input amplitudes of the input voltage, Vin, enlarge the operating range of the IGFET devices 12, 14 into a saturation region, which is also further described in conjunction with FIG. 3. Therefore, when the input voltage, Vin, is relatively large, the above-mentioned canceling effects are reduced and the integrator circuit 10 generates distortion at the output terminals 24a and 24b. In order to compensate for the non-linearities resulting from large input signals, complex circuitry (not shown) is required.